1. Field of the Invention
This invention relates to a method of designing a non-linear controller in a power system and particularly to a method of designing a static synchronous compensators (STATCOM).
2. Description of Related Art
A concept of FACTS was originally proposed by a scholar of America, N. H. Higorani, being established at the end of years 1980' of century 20 around. FACTS uses a device structured with large-power electronic components to regulate the operation of an AC electric power system. The technology of large-power electronic component becomes popular and the component is cheap day by day. Besides, the mechanism of operation of electric power system control trends to market requests so that the system operation is more complicated and variable. In order to satisfy the technical economical requirements in all aspects for participants in the market, the electric power system must be capable of self-adjustment.
STATCOM is a shunt control device in FACTS. Currently, there are many STATCOM practically operating in the power system. The power system embeeded with STATCOM can improve the stability of power system; next, it may also provide the reactive power compensation for the weak bus, and increase the capacity of of a power transmission line.
With reference to FIG. 1, a STATCOM is installed in shunt with Bus 102 to regulate the voltage of bus 102 and increase the stability of the power system. The STATCOM comprises a shunt transformer 105, a shunt converter 106, and a DC coupling capacitor 107. The DC coupling capacitor 107 stores a DC voltage and connects to the shunt converter 106. The shunt converter 106 is electrically connected to the shunt transformer 105. The shunt transformer 105 is connected to a terminal bus 102.
The converter 106 of the STATCOM may independently acts as an inverter or a rectifier. When the power is transmitted from an AC grid to the DC side coupling capacitor 107, the converter 106 acts as a rectifier, whereas the power flows from the DC side coupling capacitor 107 to the transformer 105, the converter 106 acts as an inverter.
The shunt converter 106 may receive energy from the power gird or release it. The energy infused into the bus 102 is converted to DC side capacitor 107 through the shunt transformer 105 and the shunt converter 106 to adjust the electric power stored in the DC coupling capacitor 107 so that a voltage value in the DC coupling capacitor 107 is constant.
With reference to FIG. 2 illustrating the overall structure of system STATCOM, STATCOM is shunt on the bus 102, and a resistor R 114 and an inductor L 115 are a loss resistor and a loss inductor, respectively, that are equivalent in the transmission line; a resistor R 111 is a leakage resistor of the transmission line and transformer, which is equivalent; an inductor L 112 is a leakage reactance; Sa, Sb, Sc, Sa, Sb, and Sc, six IGBT switches stand for the shunt converter 106; a capacitor C 113 is a coupling capacitor 107 that is equivalent; a resistor Rdc 114 stands for a loss when the IGBT switches run. The system status will vary with the changable load 103. Normally, through a switching circuit, the rectifier converts the DC voltage to AC voltage Vs that may be controlled by the proper switching of IGBT.
The DC voltage of capacitor 107 is maintained at desired value by controlling the shunt converter 106. A key point is the way of controlling the shunt converter 106 so as to satisfy the goal of control, without excessive harmonics that affects the quality of electric power and the feedback control signal. In a conventional method of controlling STATCOM, a proportional-integral (PI) voltage regulator is used to generate adequate control signals for controlling the shunt converter 106. In practice, the STATCOM circuit is high non-linear and the linear control theory cannot satisfy its dynamic characteristics.
In a prior art, proposed on July 1993. by C. S. de Araujo and J. C. Castro, “Vectors analysis and control of advanced static VAR compensators,” IEE Proceedings-C, vol. 140, no. 4, pp. 299-306, a decoupled model is provided for designing the STATCOM on the basis of PI controller. In a prior art, proposed on October 2004 by D. Soto and R. Pena, “Nonlinear control strategies for cascaded multilevel STATCOMs,” IEEE Trans. on Power Delivery, vol. 19, no. 4, pp. 1919-1927, a feedback linear theory is used to convert a nonlinear model to a linear model for designing a STATCOM controller; the key art is to eliminate a nonlinear element of a non-STATCOM model and add expected dynamic characteristics. It is provided not only with an advantage of simple design of a linear model, but also with the dynamic characteristics of a nonlinear model that remains.